Engines have been widely used in vehicles, ships, small power generators, and the like, and attempts to improve an efficiency of the engine are continuously conducted. In an engine, a large quantity of heat is generally discharged as waste heat, and a waste heat recovery system for increasing entire efficiency of the engine by recovering the waste heat as energy has been developed.
The waste heat recovery system is configured to recover the waste heat of the engine as energy, convert the recovered energy into electric energy or mechanical energy, and utilize the electric energy or the mechanical energy in the engine, other electrical accessories, or the like, of the vehicle.
As a typical implementation technology of the waste heat recovery system, a Rankine cycle waste heat recovery system of recovering the waste heat of the engine as energy is employed in conjunction with a Rankine cycle engine. The Rankine cycle includes a circulation path through which a working fluid is circulated, and the circulation path of the Rankine cycle is provided with a boiler for heating and evaporating the working fluid by the waste heat (heat of an exhaust gas and/or heat of an exhaust gas recirculation (EGR) gas) of the engine, an expander for expanding the working fluid supplied from the boiler to generate rotational power, a condenser for condensing the working fluid discharged from the expander, and a pump for circulating the working fluid on the circulation path.
Meanwhile, water, ethanol, refrigerant, or the like, has been used as the working fluid of the Rankine cycle engine. In the case in which the working fluid is leaked, various risks may result, and in the case in which the working fluid remains in the circulation path or a main component, damage to the main components may occur due to freezing, or the like.
For example, since water has a volume that expands when frozen at a temperature of 0 degrees or less, in the case in which the water is frozen in a main component such as a heat exchanger, the main component may be damaged, and when ethanol is leaked, there a risk that fire may result. When the refrigerant is leaked, various problems may occur due to toxicity, a high global warming potential (GWP), and the like, of the refrigerant.